The present invention relates to a composite barrier laminate for a carton for citrus beverages including juices and other liquids. More particularly, it relates to a composite barrier laminate for a carton for citrus juices that extends the shelf life of the citrus juice by inhibiting the proliferation or growth of microorganisms. The composite barrier laminate has a barrier skin coat layer that is impervious to the antimicrobial agent D-limonene so that it prevents the migration of D-limonene from the citrus juice.
The present invention also relates to a method for extending the shelf life of a citrus juice stored in a paperboard carton. The method comprises preventing the antimicrobial agent D-limonene present in the citrus juice stored in the carton from migrating into the carton structure by making the innermost layer of the carton structure impervious to D-limonene. The present invention provides a method for preserving the antimicrobial properties present in citrus juice that is stored in a paperboard carton or container by preventing the migration of D-limonene from the citrus juice by storing the citrus juice in a carton formed from a laminate having an innermost layer that is impervious to D-limonene.
The prior art believed that flavor degradation in citrus juice and other citrus beverages during storage in the preferred polyethylene coated cartons was caused by the loss of essential "flavor" oils. Accordingly, laminate structures that prevent the loss of "flavor" oils in order to avoid flavor degradation are now known in the art.
The present invention unexpectedly found that with the growth of microorganisms, such as, for example, yeast, mold and bacteria, flavor changes or defects in a citrus juice occur. The present invention also found that D-limonene in citrus juices, such as orange juice, acts as a strong antimicrobial agent that inhibits the proliferation of microorganisms which, in turn, extends the shelf life of the juice. It is believed that when D-limonene migrates into and is absorbed in the polyethylene coating of a carton, the microbial growth in the citrus juice after eight weeks can be one hundred times greater than its initial condition.
Heretofore, the useful shelf life of a citrus juice has been limited to a few weeks. It is now believed that the principal reasons are flavor loss due to microbial growth and loss of nutritional value due to the oxidation of ascorbic acid (vitamin C) below the regulation level of 30 mg ascorbic acid per 100 ml. The loss of ascorbic acid can be prevented for seven or eight weeks by using oxygen barrier layers such as polyamide, ethylene vinyl alcohol (EVOH) or polyethylene terephthalate (PET). Nevertheless, whether the juice is held in non-oxygen barrier or oxygen barrier cartons, flavor loss can become noticeable within about two weeks because as much as thirty percent of the D-limonene in the citrus juice may be absorbed into the heat seal coating layer within a few days. This is known as scalping.
Glass containers and molded plastic containers have been used to store and transport juice. However, both have a number of disadvantages, including shipping costs for the empty containers, disposal problems, breakage and weight problems.
To obtain the advantages of comparatively low container weight, ease of sealing and opening, ease of disposal and low shipping volume for empty containers, since such containers can be shipped in a collapsed condition, containers based on a paperboard substrate are the current choice for marketing citrus juice and other products. These cartons are known in the trade as "gable top" containers or cartons. Such a carton is described in U.S. Pat. No. 3,120,333. These gable top cartons use low density polyethylene as the preferred food-contact polymer.
It has, however, been found that low density polyethylene (LDPE) has a very high absorption and migration capacity for D-limonene. Unless a D-limonene barrier layer is part of the container construction, migration can result in as much as a 60% loss of D-limonene before the expiration of the container's sale date.
The prior art has provided laminates having a metal foil layer to retain the vitamin content and flavors in the juice for a substantial period of time, about ten weeks. The skin coat layer is not, however, the metal foil layer. Also, these laminates do not address the issue of inhibiting the migration or the resultant loss of the antimicrobial agent from the citrus juice. Further, metal foil laminate containers are expensive when compared to non-metal foil laminate containers, and are prone to develop pin holes that seriously affect their ability to contain liquids.
While considerable efforts have been devoted to finding the best layer structure in a barrier laminate, such efforts have failed to appreciate the superior and unexpected results of longer shelf life which is achieved by providing a thin skin coat layer that is impervious to D-limonene to prevent the migration of D-limonene from the citrus juice.
For example, U.S. Pat. No. 4,513,036 describes a laminate useful in making a paperboard based carton for juices, that provides an effective barrier to the migration of essential oils and/or flavors, and does not contain a metal foil. This laminate has a paperboard substrate, a polypropylene polymer layer coated thereon and a polyolefin layer applied onto the polypropylene polymer layer.
A laminate that provides an effective barrier to oxygen and, thus, helps retain vitamin C in the juice stored in the carton is described in U.S. Pat. No. 4,777,088. This laminate comprises from the outer surface to the inner surface which would contact the juice, a paperboard substrate, a nylon layer coated directly on the paperboard substrate, a layer of poly(ethylene-co-methacrylic acid) partially neutralized with sodium and zinc cations (Surlyn) directly overlying and in contact with the nylon layer, and an inner polyolefin layer in contact with the Surlyn layer. The nylon barrier layer helps retain vitamin C and, in addition, essential oils and flavor as does the laminate in U.S. Pat. No. 4,513,036.
Likewise, other barrier laminates that are capable of excluding oxygen and preventing the loss of oils and/or flavors are described in U.S. Pat. Nos. 4,977,004, 4,977,009, 4,950,510, 4,861,526, 4,835,025, 4,806,399, 4,753,832, 4,701,360 and 4,698,246. In U.S. Pat. Nos. 4,701,360, 4,861,526 and 4,698,246, both sides of a paperboard substrate are first flame treated and a layer of low density polyethylene is applied to the outside surface. To the surface of the paperboard that becomes the inside surface of the carton, a layer of low, density polyethylene is initially applied directly to the paperboard, followed by a nylon barrier layer that is applied to that interior low density polyethylene layer with a bonding tie layer between the nylon and the polyethylene. Lastly, an innermost skin layer is applied to the nylon with another tie layer to improve the adhesion of the layers and to help in heat sealing. This skin layer is made of ethylene vinyl alcohol copolymer.
U.S. Pat. No. 4,948,640 provides five embodiments for a heat sealable barrier laminate. Each embodiment provides a substantial barrier to loss of vitamin C over an extended shelf life of the carton, namely about eight weeks. Each embodiment further provides that the skin coat layer is a thin layer of a low density polyethylene polymer.
Not one of these patents recognizes that the cause of flavor degradation in citrus juice, such as orange juice, is the proliferation of microbiological organisms or microorganisms that occurs when the D-limonene migrates out of the juice. Accordingly, these patents fail to provide a method for inhibiting the loss of the antimicrobial agent in the citrus juice contained in the carton or, moreover, appreciate that a substantial increase in shelf life can be accomplished by inhibiting the proliferation of the microorganisms by providing an innermost layer that is made of a material impervious to the antimicrobial agent D-limonene thereby retarding the loss of D-limonene from the citrus juice.
Thus, the present invention provides a barrier skin coat that will prevent the migration or loss of D-limonene from the citrus juice, inhibit the growth of microorganisms, and the rapid loss of flavor due the the growth of microorganisms.